Nonagon

Regular nonagon (enneagon)
A regular nonagon (enneagon)
Type	Regular polygon
Edges and vertices	9
Schläfli symbol	{9}
Coxeter diagram
Symmetry group	Dihedral (D9), order 2×9
Internal angle (degrees)	140°
Dual polygon	self
Properties	convex, cyclic, equilateral, isogonal, isotoxal

In geometry, a nonagon /ˈnɒnəɡɒn/ (or enneagon /ˈɛniːəɡɒn/) is a nine-sided polygon or 9-gon.

The name "nonagon" is a prefix hybrid formation, from Latin (nonus, "ninth" + gonon), used equivalently, attested already in the 16th century in French nonogone and in English from the 17th century. The name "enneagon" comes from Greek enneagonon (εννεα, "nine" + γωνον (from γωνία = "corner")), and is arguably more correct, though somewhat less common than "nonagon".

Regular nonagon

A regular nonagon is represented by Schläfli symbol {9} and has internal angles of 140°. The area of a regular nonagon of side length a is given by

$A = \frac{9}{4}a^2\cot\frac{\pi}{9}=(9/2)ar = 9r^2\tan(\pi/9)$

$= (9/2)R^2\sin(2\pi/9)\simeq6.18182\,a^2,$

where the radius r of the inscribed circle of the regular nonagon is

$r=(a/2)\cot(\pi/9)$

and where R is the radius of its circumscribed circle:

$R = \sqrt{(a/2)^2 + r^2 }=r\sec(\pi/9).$

Construction

Although a regular nonagon is not constructible with compass and straightedge (as 9 = 3², which is not a product of distinct Fermat primes), there are very old methods of construction that produce very close approximations.^[1]

It can be also constructed using neusis, or by allowing the use of an angle trisector.

Nonagon, an animation from a neusis construction based on the angle trisection 120° by means of the Tomahawk

The following is an approximate construction of a nonagon using a straightedge and compass.

Another animation of an approximate construction

Downsize the angle JMK (also 60°) with four angle bisectors and make a thirds of circular arc MON with an approximate solution between angle bisectors w3 and w4. Straight auxiliary line g aims over the point O to the point N (virtually a ruler at the points O and N applied), between O and N, therefore no auxiliary line. Thus, the circular arc MON is freely accessible for the later intersection point R. $\scriptstyle\angle{}$ RMK = 40.0000000052441...° 360° ÷ 9 = 40° $\scriptstyle\angle{}$ RMK - 40° = 5.2...E-9° Example to illustrate the error: At a circumscribed circle radius r = 100,000 km, the absolute error of the 1st side would be approximately 8.6 mm. See also the calculation (Berechnung, German).

Symmetry

Symmetries of a regular enneagon. Vertices are colored by their symmetry positions. Blue mirrors are drawn through vertices, and purple mirrors are drawn through edge. Gyration orders are given in the center.

The regular enneagon has Dih₉ symmetry, order 18. There are 2 subgroup dihedral symmetries: Dih₃ and Dih₁, and 3 cyclic group symmetries: Z₉, Z₃, and Z₁.

These 6 symmetries can be seen in 6 distinct symmetries on the enneagon. John Conway labels these by a letter and group order.^[2] Full symmetry of the regular form is r18 and no symmetry is labeled a1. The dihedral symmetries are divided depending on whether they pass through vertices (d for diagonal) or edges (p for perpendiculars), and i when reflection lines path through both edges and vertices. Cyclic symmetries in the middle column are labeled as g for their central gyration orders.

Each subgroup symmetry allows one or more degrees of freedom for irregular forms. Only the g9 subgroup has no degrees of freedom but can seen as directed edges.

Graphs

The K₉ complete graph is often drawn as a regular enneagon with all 36 edges connected. This graph also represents an orthographic projection of the 9 vertices and 36 edges of the 8-simplex.

8-simplex (8D)

Pop culture references

They Might Be Giants have a song entitled "Nonagon" on their children's album Here Come the 123s. It refers to both an attendee at a party at which "everybody in the party is a many-sided polygon" and a dance they perform at this party.^[3] Slipknot's logo is also a version of a nonagon, being a nine-pointed star made of three triangles.

Architecture

Temples of the Baha'i Faith are required to be nonagonal.

The U.S. Steel Tower is an irregular nonagon.

See also

• Enneagram (nonagram)
• Trisection of the angle 60°, Proximity construction

References

1. J. L. Berggren, "Episodes in the Mathematics of Medieval Islam", p. 82 - 85 Springer-Verlag New York, Inc. 1st edition 1986, retrieved on 11 December 2015.
2. John H. Conway, Heidi Burgiel, Chaim Goodman-Strauss, (2008) The Symmetries of Things, ISBN 978-1-56881-220-5 (Chapter 20, Generalized Schaefli symbols, Types of symmetry of a polygon pp. 275-278)
3. TMBW.net

• Weisstein, Eric W., "Nonagon", MathWorld.

External links

• Properties of a Nonagon (with interactive animation)